To control the noise from air handling units (AHUs), duct active noise control (ANC) systems are starting to be employed in air distribution systems. An ANC system basically requires the sensing of the noise associated with the fan for distributing air, producing a noise canceling signal and determining the results of the canceling signal so as to provide a correction signal to the loudspeaker. There is a time delay associated with sensing the noise and producing a canceling signal. This time delay necessary for the canceling to take place equates to the distance in the system required between the reference, or input, noise sensor and the loudspeaker. Additional space is required between the loudspeaker and the error sensor which also equates to a distance in the system.
A centrifugal fan discharges into a scroll which provides an expanding flow path for the air passing from the impeller into the scroll. The flow path, typically, is expanding radially but may also be expanding axially. It is common to characterize a fan by the angle of the slope of a plot of the variable scroll radius (R) vs the angular or circumferential extent (r.theta.), where r is the radius of the fan and .theta. is the angular extent). In conventional designs, the annular extent of the flow path is less than 360.degree., with 300.degree. being typical. The reasons for this angular extent is the diffusing of the air flow as well as minimizing the space and material needed to make the scroll.